1. Field of Invention
The present invention relates to method for manufacturing a metal oxide semiconductor transistor. More particularly, the present invention relates to a method for manufacturing a metal oxide semiconductor transistor having a raised source/drain.
2. Description of Related Art
Progress in semiconductor fabrication technologies has made it possible to fabricate semiconductor devices at the ULST level. However, even though decreasing the size of devices increases the density and operation efficiency of the integrated circuit, such a decrease is still accompanied by a short channel effect, which decreases the device performance. As the size of devices decrease, the major parameters which may affect the electric property of the transistor include: the length of the channel, saturation of speed, resistance of the source/drain region, penetration of electrons, drain induced barrier lowing (DIBL), geometric shape and etc.
When the channel length is shorter than the junction depth of the source/drain, major charges on the channel located below the gate electrode transfer into the junction depletion region of the source/drain and only minor charges transfer into the gate electrode; hence, the threshold voltage is decreased. Conventionally, in order to minimize the variation of the threshold voltage, an extension structure of the source/drain region is formed on the substrate corresponding to the source/drain region of the transistor and an epitaxial layer functioning as a raised source/drain is formed on the source/drain. The high temperature, above about 800 degrees centigrade, at which the epitaxial layer is formed redistributes the dopants previously doped into the extension structure of the source/drain. Therefore, it is hard to control the outline of the shallow junction. Furthermore, the large variation of the threshold voltage and the decreasing of the device reliability are unavoidable.